Currently, exhaust lines for motor vehicles are provided with pollution control means and in particular devices for purification of exhaust gases, such as catalytic converters or particulate filters. These purification devices comprise, in a casing, a substrate which is permeable to exhaust gases, and which is optionally impregnated with catalytic substances which facilitate the conversion reaction of some polluting substances which are contained in exhaust gases.
Purification devices are commonly received in housings which are provided below the floor of the vehicle or in the engine compartment thereof. This housing sometimes has a small height and/or volume, with the result that it is complex to arrange the purification device at that location.
The substrates included in these purification elements are generally cylindrical having a circular cross-section. The length of the substrate measured between the upstream and downstream faces is often very much greater than the diameter of the upstream and downstream faces. In the case of a purification element which is placed below the floor, the substrate is arranged with the length thereof arranged horizontally so that the height of the cavity which receives the catalytic purification device must be greater than the diameter of the upstream and downstream faces of the substrate. In order to allow a purification element to be placed in a cavity having a small height, it was envisaged to use catalytic substrates which have a very small length and in contrast have very extensive upstream and downstream faces. The length is referred to as the thickness and these substrates are referred to as “pancakes” since they have a shape which is similar to a disc.
These substrates are arranged with their thickness very slightly inclined relative to the vertical. In order to allow the gases to flow, the inlet channel and the outlet channel of the catalytic purification device open with a slight incident angle practically tangentially relative to the upstream and downstream faces of the filter.
This type of catalytic purification device has low levels of efficiency in terms of pollution control, in particular owing to the unfavourable ratio between the small thickness of the filter and the large extent of the upstream and downstream faces, which limits the contact between the catalytic materials and the exhaust gases.